The present invention relates to fixturing for clamping a workpiece during machining in general. In particular it concerns universal expanding clamps for clamping an internal surface of a workpiece and universal contracting clamps for clamping an external surface of a workpiece during grinding, milling, boring or other machining procedures.
It is known to use an expanding mandrel for clamping against an internal surface of a workpiece for grinding, turning and inspection operations. Conventional expanding mandrels are of many types, but may be classified by their mechanism into two categories: expandable-sleeve-type mandrels and insert-type mandrels. Expandable-sleeve-type mandrels have an expandable sleeve axially aligned with a clamping taper so that axial movement of the sleeve relative to the clamping taper expands the sleeve to clamp the internal diameter of the workpiece. Insert-type mandrels have a rigid body with a number of radially moveable inserts which are pushed outward by axial movement of a central clamping taper or a cylindrical piston with inclined slots. These two types of mandrel are in most cases interchangeable. When reference is made below to expandable-sleeve-type mandrels it is to be understood as referring also to insert-type mandrels.
It is also known to use a collet chuck for clamping against an external surface of a workpiece for similar operations. Collet chucks work similarly to expandable-sleeve-type mandrels, with a compressible sleeve (known as a collet) axially aligned within an internally tapered clamping taper. Collet chucks are generally analogous in their structure and function to expanding mandrels.
The use of expanding mandrels is potentially an important clamping technique since it provides access to the entire outer surface of a workpiece, allowing efficient use of modem machining methods. However, expanding mandrels are currently only used in a small proportion of grinding, turning and inspection applications, and are not used for static fixturing at all.
Reasons for the limited use of expanding mandrels may be divided into two categories: first, economic considerations mitigating against their use; and second, practical limitations of expanding mandrel designs.
Referring first to economic considerations, there are many variables involved in selection of the appropriate expanding mandrel for a particular application. These include: the diameter of the sleeve; the length of the sleeve; the length of the mandrel; the position of a stop along the mandrel; the type of mounting used; and, the method of tightening. Dealing now with each of these variables separately:
(i) The diameter of the sleeve must be chosen to fit the diameter of the internal surface to be clamped. A typical conventional expanding mandrel only expands radially by a fraction of a millimeter, and even insert-type mandrels which are somewhat more expandable have a range of expansion of about one millimeter radially. Thus, for example, to be able to clamp workpieces with a range of diameters from 60 mm. up to 110 mm. typically requires between 80 and 100 different diameters of sleeve. PA1 (ii) The length of the sleeve must also be chosen to fit the internal surface to be clamped. When used with a short workpiece, the sleeve should not be more than 50% longer than the surface to be clamped. For longer workpieces, the sleeve should clamp along a large proportion of the available internal surface so as to support the workpiece securely. PA1 (iii) The length of the mandrel must be chosen so that the sleeve is positioned at the required distance from the mandrel mounting. This is important to ensure access for machining tools to the surface to be machined. PA1 (iv) If a stop is being used, its position along the mandrel must be chosen so that the sleeve is inserted into the workpiece to the right depth. PA1 (v) The type of mounting used also varies between applications. A workpiece with a through-hole may be mounted between centers. A mandrel with a morse-taper may be mounted directly in a machine spindle. For increased stability, or for high torque applications, a mandrel with a flange mounting is required. PA1 (vi) Mandrels which use several methods of tightening are available. The simplest use manual (percussion) tightening. For clamping a workpiece with a through-hole, a mandrel tightened by a clamping screw or nut may be used. Other mandrels have an internally threaded clamping ring, and for power-assisted clamping and automated systems a draw bolt design is required.
Because of all these variables, each expanding mandrel is specific to a very small number of applications. A workshop will therefore commonly need to purchase a large number of mandrels to cover a range of applications, or will have to order a new one for each workpiece to be clamped. Each expanding mandrel is made up of complicated high precision components made from hardened steel and requiring precise internal and external dimensions, high concentricity and slotting of the sleeve to allow expansion. Since each expanding mandrel is therefore time-consuming to produce and costly to buy, the use of expanding mandrels is often abandoned in favor of cheaper alternatives.
Referring now to the practical limitations of expanding mandrel designs, these relate primarily to the clamping action of the expandable sleeve. Conventional expanding mandrels have a cylindrical expandable sleeve which is only suited to clamping a predominantly cylindrical internal surface. If the internal surface of a workpiece has pans with different internal diameters, the sleeve will only clamp either the pan with the smallest diameter or the pan nearer the opening. In many cases the clampable surface will be too small or too close to one end of the workpiece to clamp it effectively against the forces produced during machining and to ensure accurate alignment along the mandrel. Where walls of a workpiece are thin, clamping against a small surface causes deformation, damaging the workpiece. Machining of workpieces in which the external surface is eccentric relative to the internal surface cannot be easily achieved using conventional expanding mandrels.
An additional practical limitation of expanding mandrel designs relates to angular alignment of the workpiece. Many conventional expanding mandrels have a flange-like projection, called a stop, which fixes the position of the workpiece along the mandrel. However, no provisions exist for locating the workpiece in a defined angular position. This limits the usefulness of expandable mandrels for static fixturing for which the precise translational and angular position of the workpiece must be known.
There is therefore a need for universal expanding mandrels which will allow one mandrel to be used for many different workpieces, will enable clamping of multiple internal diameters, will allow easy machining of eccentric workpieces and will enable precise angular alignment of a workpiece about the mandrel.